Transformable, applicable material and methods for using same for optical effects

ABSTRACT

Optical elements and methods of forming optical elements are described. In one example, a method applies a photosensitive ink to a substrate and exposes the ink to create an optical element, such as an optical lens or holographic recording medium.

This application claims priority to co-pending U.S. Provisional PatentApplication No. 60/603,143, which was filed on Aug. 19, 2004.

FIELD OF THE INVENTION

The present invention relates to optical elements such as lenses.

BACKGROUND OF THE INVENTION

Prior art lenses are created by grinding glass into a desired shape toachieve the desired optical element. Grinding is a mechanical processwhich is expensive and time consuming and requires great care andprecision if an optical element is to be formed within a small toleranceof certain optical properties (e.g. its focal length, etc.). It isdesirable to provide alternative techniques which are potentially easierand less expensive.

SUMMARY OF THE DESCRIPTION

Various examples of optical elements, which are formed by exposing aphotosensitive material to electromagnetic radiation, are described. Theoptical elements are generally designed to allow visible light (e.g.light having wavelengths in the range of about 400 nm to about 800 nm;such light is visible to most humans) to pass through the opticalelement or to reflect from (and/or interact with) the optical element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, in three parts, shows how information, using embodiments of theinvention, may be recorded into a photosensitive material.

FIG. 2, in three parts, shows how the information, which may be recordedin the manner shown in FIG. 1, may be retrieved from the medium.

FIG. 3 shows an example of a holographic data storage system.

DESCRIPTION

Various embodiments of this invention relate to photosensitive ink, theapplication of the ink to a substrate, the exposure of the coatedsubstrate to electromagnetic radiation (or other energy source) tocreate refractive index (or other optical) changes in it, and the effectthat the changes (e.g. a stored phase pattern such as a grating) has onincident light when subjected to thereafter. The invention also relatesto the creation (e.g. storage) of specific patterns of refractiveregions, resulting from exposure to specific patterns (such asinterference patterns) of the writing energy, that result in diffractionphase gratings, holograms, mirrors, filters, lenses, compensated lenses,data, barcodes, micro-optics, waveguides, fibers, lasers, OPOs,doublers, optical circuits, antennas, prisms, polarizers, opticalswitches, confinement structures, other optical components, and arraysof thereof. The invention further relates to methods, systems,mechanisms, and machines for creating such refractive index changesand/or phase patterns in substrates coated with photosensitive ink. Theinvention also relates to mechanisms for launching light into, onto,through, and off of phase patterns thusly created to produce theintended end (bulk) optical or holographic effect. The invention alsorelates to methods for encoding and storing data, holographically, ontosubstrates thusly prepared, and to methods for retrieving, reading, anddecoding the stored data. Specifically, the invention relates to anentirely new class of optical elements and components, whose performancehas traditionally only been realized in bulk form, including printableholographically stored data, that can be manufactured in part usingprinting technologies, and subsequently effected (e.g. exposed to laserlight) to impart optical performance therein. The invention cantherefore be used to create optical elements, store data, createinteresting visual effects, create covert security features and effects,to channel light and energy across distances (including curvedsurfaces), to reflect, deflect, focus, magnify, change frequency andwavelength of, correct for, and otherwise alter and control light inuseful and potentially in low cost ways.

A photosensitive ink could nominally be made from a liquid or monomerbinding or carrier agent into which small particles of 1) photosensitiveglass (such as ¹GeSiO₂, ²H₂:GeSiO₂, SiO₂, B:SiO₂, Sn:SiO₂,³Ce,Ag,F:SiO₂, soda-lime, leaded, borosilicate, oxide, non-oxide, orothers) and/or 2) photosensitive crystals (lithium niobate, SBN, orothers) and/or 3) photosensitive polymers (Dupont HRF-150, Norlandoptical cements, or others) can be mixed. The photosensitive pigmentparticles may also contain other dopants or semiconductors such as tin,boron, phosphorous, aluminum or other metals, etc, and may also beloaded with molecular hydrogen to increase their photosensitivity. Theink could also be comprised of a liquid that can undergo aphotosensitive refractive change on its own with no additional particlesor components.¹A. I. Gusarov, et, al, Optics Letters, Vol 25, No. 12, Jun. 5, 2000.²Raman Kashyap, “Fiber Bragg Gratings”, Academic Press 1999, ISBN#0-12-400560-8.³O. M. Efimov et al, Optics Letters, Vol 25, No. 23, Dec. 1, 2000.

The photosensitive ink can be applied to a substrate (paper (such as newor recycled cellulose based paper), plastic, plastic cards, glass,metal, wood, textiles, leather, etc) via a printing or coating process.The photosensitive ink can be flood coated onto a large substrate area,or selectively printed onto small-localized regions such as in thedenomination value (number) on the corner of a banknote. The ink canthen be dried (solvent evaporation) or cured by electromagneticradiation such as ultraviolet (UV) or otherwise stabilized/hardened tosecure the particles. After being printed/coated with such ink, thesubstrate can be subsequently exposed to UV (or other wavelengths suchas for instance visible, IR, or pulsed ⁴IR) causing it to undergo achange in refractive index of either the particles, the binder, or both,in the regions of exposure. {In glasses, this refractive index change isa result of various physical phenomena including the formation of colorcenters and the densification of the material, which leads to a volumechange in the material, which leads to stress induced birefringence. Incrystals it can be the result of the photorefractive effect and thecreation of space charge fields⁵. In polymers it can result from thecreation of new bonds as monomers are turned into polymers in thematerial⁶.}⁴D. Homoelle et. al (Nick Borelli), Optics Letters, Vol 24, No. 18, Sep.15, 1999.⁵See pertinent publications by Jeff Wilde.⁶Demetri Psaltis, Geoffrey Burr, IEEE Computer, February 1998,0018-9162/98.

Exposure of the printed photosensitive region and the resultant creationof a refractive index change on the substrate is intended to impartinformation (holographic data storage⁷), visual effects (holograms oriridescence patterns for instance), or other optical phenomena(diffraction gratings, filters, lenses, or mirrors⁷See publications on holographic data storage. for instance) on thesubstrate in the form of a phase change or a phase grating. Such a phaseor holographic grating could take the form of a simple periodic gratingfor diffracting light (say for instance to make an anti-copy feature ona document), or a more complex structure such as a visible holographicfeature similar to those seen on credit cards. The phase grating couldbe constructed to produce structural patterns of different refractiveregions such as those found on butterfly wings^(8,9,10,11) producinghighly iridescence effects. It could also be used to create lenses,mirrors, fibers, waveguides, Fresnel lenses, arrays of lenses, microlenses, micro lens arrays, and light management films for displaysincluding those made from micro lens arrays, and a host of othertraditionally bulk optical or discrete optical components.

The invention can also be used to create security and informationfeatures on documents (and other substrates) as a means of establishingauthentication. For instance, personal, corporate, governmental,ownership, title, dated, and other data including photographs,fingerprints, aliases, etc., can be digitally stored as a hologram on apassport or visa in this manner. The information can be later read outfrom the document by a reader designed to illuminate the selected regionproperly and decode the diffracted pattern¹².⁸P Vukusic et al, Nature Vol 404 March 2000.⁹P. Vukusic et al, Nature Vol 410 March 2001.¹⁰Chris Lawrence, Applied Optics, Vol. 41, No. 3, Jan. 20, 2002.¹¹P. Vukusic et al, Proc. R. Soc. Lond. B. (1999) 266, 1403-1411.¹²Demetri Psaltis, Geoffrey Burr, IEEE Computer, February 1998,0018-9162/98.

The refractive pattern can be in the form of alphanumeric characters ora bar code that has been scanned in with a laser, or it can be in theform of an interference pattern that is created by intersecting coherentlaser beams. Digital information can be encoded onto one of the beams bysending it through a spatial light modulator prior to the intersectionof the beams that causes the interference pattern¹³. Information that isstored this way can be read back by using a laser beam to reconstructthe pattern. Methods include phase contrast microscopy and diffraction,both of which can be used to reconstruct phase information from agrating in either transmission of reflection mode.

Certain embodiments of the invention will include one ore more of thefollowing components:

-   -   1.) An ink that will respond to electromagnetic radiation by        changing its refractive index. The ink consists of a binding        agent with particles of glasses (such as GeO₂:SiO₂ glass that        have been loaded with hydrogen). When exposed to UV radiation of        the appropriate wavelength and intensity, the refractive index        of the glass particles can be changed.    -   2.) Method for applying ink to substrate—such as printing,        coating.        ¹³See publications on holographic data storage.    -   3.) Method for optically (or otherwise) altering the refractive        index or other optical properties in select regions and in        controllable ways on the printed or coated substrate.    -   4.) Method for optically storing information (e.g. digital        information) on substrate materials coated with said ink in the        form of, for example, a grating or hologram. (Holographic data        storage.)    -   5.) Method for optically retrieving stored information.        (Holographic data storage.)

Optical fibers made from doped mixtures of germanium dioxide and silicondioxide glasses form the backbone of the telecommunications industry.Typically produced from MCVD performs which have been heated and pulled,these fibers contain a core and a cladding that enables light at awavelength of ˜1.5 microns to propagate through them for thousands ofkilometers. Though the fiber core material is extremely transparent,small absorption losses eventually cause degradation in the signalsrequiring them to be amplified. The erbium doped fiber amplifier is alsoan optical fiber that is pumped at 980 nm to generate light at 1.5microns via the ⁴F_(13/2) to ⁴I_(15/2) transition in Er³⁺. Theseamplifiers build up gain by reflecting the 1.5 micron emission from oneend of the fiber to the other using integrated gratings as mirrors,thereby stimulating additional light at this wavelength. The internalintegrated mirrors are fabricated by physically recording fiber Bragggratings, into the ends of the fibers using ultra-violet light from alaser.

Hydrogen loaded germanosilicate glasses display a photosensitivity thatenables UV light to induce defects in the fiber core that changes therefractive index by large amounts (Δn=0.03). These changes can be madeto be permanent resulting in a grating that lasts for a very long time,even at elevated temperatures (500 C). The material system and processused to produce Bragg gratings in fibers, namely hydrogen loaded dopedGeSi glasses, can be readily made into small particles for incorporationinto printing ink, which will enable UV sensitive refractive regions tobe integrated onto documents and other substrate materials. By printingphotosensitive inks onto substrates, information in the form of serialnumbers, dates, bar codes, fingerprints, photographs, video clips andmovies, large volumes of information, etc can be stored in a phasegrating and subsequently read out using light and optical detectors.Information encoded onto substrates in this manner can be completelycovert, if desired, providing a means of securing documents againstcounterfeiting and forgery. Alternatively, it can be overt creatingvisual holographic effects including full color holograms. FIG. 1 showshow a coherent reference beam (planar wavefront) and an object beam(non-planar wavefront) can be intersected in a layer of photosensitiverefractive recording media to write a phase grating in the material. Theintensity peaks created by the interference pattern induce refractiveindex changes in the material, while at the nodes (intensity nulls) thematerial retains its original, unaltered index. A phase grating that canbe permanently stored in this manner will yield its stored pattern uponre-illumination by a planar wavefront (see FIG. 2).

Certain embodiments of this invention have several components, whichcontribute to its uniqueness. They include a) the photosensitive opticalparticles such as those made from doped GeSi glass compositions andsubsequently loaded with molecular hydrogen, b) the carriers or binderswhich have been loaded with a sufficient amount of the photosensitiveoptical particles to form an ink or coating (the “inks”), c) aprinting/coating process for applying the photosensitive inks, d) anencoding process for creating and storing phase gratings in thesubstrate region on which the photosensitive material has been applied,e) a method for reading out digital information that has been stored inthe grating, and f) for optical elements that do not require readout, ameans of using them such as in an optical system (pair of eyeglasses,camera, etc).

a.) Photosensitive Optical Particles

The photosensitive optical particles can be fabricated from silicate andoxide glasses such as doped compositions of germanosilicate glass(GeO₂:SiO₂) that have been hydrogen loaded. Several such glasscompositions are currently used for telecommunications fibers with borondoped high germania content (˜30%) producing some of the highest Δn. Theparticles can be made small enough to enable printing for the requiredprinting process (intaglio, offset, flexographic, flood coating, etc),which is typically between 100 nm and 100 microns in mean diameter. Theparticles can be fabricated via precipitation methods, spraying methods,grinding methods, or any method that provides the requisite particlesize and composition, including grinding up fibers or bulk glass. Theparticles can also be incorporated with (molecular) hydrogen, possiblyusing the cold, high-pressure soaking method described in section 2.4.4of Raman Kashyap's book titled Fiber Bragg Gratings, and assortedreferences. Once infused with hydrogen, the particles can be stored atlow temperature (e.g. −70° C.) to extend the duration of theirsensitivity, until they are ready to be mixed with the ink/carrier andprinted/applied. Molecular hydrogen will remain in the glass particlesfor a period of time at room temperature providing sufficient time tomix the inks, print the substrate, and encode the gratings. After aperiod of time at standard temperature and pressure, particles that havenot been exposed to UV will lose hydrogen through diffusion, reducingtheir sensitivity to the writing wavelengths. This helps to prevent newgratings from being subsequently written over the original gratings,thereby altering or erasing the intended content. If necessary thisprocess can be accelerated by heating the documents after theinformation has been encoded. Photosensitive particles that have losttheir infused hydrogen due to diffusion out of the particles, forinstance, can be “reconstituted” (so to speak) by exposing them onceagain to the process of hydrogen loading. This sensitizing process canhappen several times if need be as the hydrogen causes no adversereactions in the glass.

It should be noted that the use of hydrogen loaded germanosilicate glassparticles is only one example of a photosensitive particle that could beused. Other compositions of glasses including doped glasses andsemiconductor-doped glasses can also be used. Additionally, crystals,such as lithium niobate and SBN, and even particles of polymers(Dupont's HRF-150 for instance) could also be used to provide thisfeature. Hydrogen loaded Ge silicates have been identified as a materialthat can provide enhanced photosensitivity for a reasonable shelf lifeunder appropriate storage conditions, and which then looses its enhancedphotosensitivity shortly after being written (again under properconditions), but which provides permanent gratings.

Additionally, since various transition metals, heavy metals,semiconductors, rare earth halides, gases, etc. (H, Li, Na, K, Rb, Cs,Fr, Be, Mg, Ca, Sr, Ba, Ra, Sc, Y, La, Ac, Ti, Zr, Hf, V, Nb, Ta, Cr,Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd,Hg, B, Al, Ga, In, Tl, C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, O, S, Se,Te, Po, F, Cl, Br, I, At, He, Ne, Ar, Kr, Xe, Rn, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) can be added to glasses to impart colorand other sensitizing properties, pigmented particles can also be usedas the photosensitive materials.

b.) Inks and Coatings

Photosensitive inks can be fabricated by mixing the photosensitiverefractive pigments with inks and other fluid media that can dry orotherwise solidify or harden. Standard inks can be used includingpigmented inks and/or clear colorless inks. Efforts can be made tominimize the overlap of absorption lines or curing frequencies in theink so that they do not substantially overlap the UV writing frequenciesof the particles, typically around 190-300 nm, but sometimes in thevisible or IR spectral regions. If H₂:GeSiO₂ particles are used, mixinginto the inks can be done rapidly to minimize the time the particles areexposed to high (room) temperatures prior to encoding, as the hydrogenwill begin to diffuse back out of them. Since storing temperatures forthe hydrogen loaded particles are at ˜−70° C., it is likely that theywill have to be brought to somewhat of an elevated temperature (˜ roomtemperature) prior to integration into the ink. (Prior to hydrogenloading, the particles do not require refrigeration.) The ink can besolidified by drying, elevated temperature thermal curing, UV exposure(e.g. to provide polymer cross linking) taking care that the UVwavelength does not induce unwanted and premature refractive changes inthe pigment. Particles can be mixed into the inks through processes suchas ball milling, vortex mixing, stirring, etc.

Though the use of photosensitive particles has been suggested as theprimary means of formulating the ink, photopolymers that provideoptically induced refractive changes can also be used, without the needto add additional particles. Such materials, once solidified on thesubstrate either by evaporative hardening, thermal curing, UV curing, orother means, would then be subject to the same types of exposureconditions to store phase gratings as refractive index changes.

c.) Printing

Once the photosensitive particles have been integrated into ink, or theink has been prepared, the ink can be printed onto an appropriatesubstrate material (cellulose based paper, reflective surfaces, mylarand plastics, metals, etc) using any printing process. It may be helpfulto let the ink dry/cure before the gratings are exposed to the encodingUV source, to ensure that the particles are firmly fixed in position.This helps to make sure that once the particles are exposed to UV andtheir refractive index is permanently altered, that they stay in thepositions required to provide the diffractive response that the readinghardware will be looking to detect. If the modified particles drift ormove after the encoding process, then the phase grating pattern may bealtered uncontrollably and the recorded information can be lost ordegraded.

The photosensitive optical regions can be printed over top of otherprinted, colored, or altered regions of the substrate to disguise theirpresence or effect the appearance of the document in some desired way.It may also be possible to print over top of the printed photosensitiveregions, either before or after the phase grating has been encoded. Theprimary concern here is that pigments or other constituents of theoverlapping inks not absorb too much of the wavelengths (of UV) used towrite and read the gratings. Additionally, it may also be possible toincorporate other pigments directly into the ink with the photosensitiveoptical particles to color them in some desired manner.

d.) Encoding of Data

Any region of the substrate thusly printed may have its refractive indexaltered upon exposure to ultra violet (or other) light (or otherelectromagnetic radiation) of the requisite wavelength, intensity, andduration. Data encoding is the process by which a specific pattern isrecorded in the printed optical area, in a manner that enables it to besubsequently read back to provide access to this information. Data canbe recorded as a phase grating via a number of mechanisms includingexposing the printed surface to an interference pattern that has beencreated by intersecting a collimated beam with a planar wavefront(reference beam) with one that has propagated through a spatial lightmodulator onto which a specific character sequence has been effectivelyimprinted as an amplitude mask (object beam). Such encoding schemes aretypically used for bulk or volume holographic data storage intended tostore tens of terabits of information per cubic centimeter. Thesesystems do not in any way address, mention, or even allude to thestorage of smaller amounts of data onto printed substrates. The opticalencoding hardware for volume holographic data storage can be applied tothis application however to write gratings into thin layers of ink whichhave been printed onto substrate. (FIG. 3 shows an example of aholographic data storage system.) Data can also be recorded by scanninga UV laser across the printed surface, using any appropriate scanningmethodology, to write information directly without the use ofinterference. Such information could be a bar code, an alphanumericcharacter sequence or a pre-calculated grating pattern, each of whichcan be read back with a plane wave reference beam to produce adiffraction pattern that is characteristic of the recorded data.Regardless of the mechanism used to impart the refractive pattern (phasegrating) onto the printed region of the substrate, it can be madepermanent and the reconstructed diffraction pattern can be used toidentify the printed object and correlate it to other information.

e.) Read Out

Reconstruction the object beam to retrieve the stored information fromthe encoded phase grating can be done in multiple ways. One example isthrough the use of diffraction, which is done by exposing the hologramto a planar wavefront (reference beam) and detecting the intensityvariations on a spatially pixilated detector (see FIG. 3). This can bedone in reflection or transmission mode, and is independent on whetherthe information was stored holographically using an interference patternor was written directly by a scanned laser. Diffraction or holographicbased readout methods will differentiate a low level signal (0.01) fromno signal (0.0) at all, typically from 0 to 0.01. Another way is throughthe use of interference or phase contrast microscopy, which willdifferentiate a totally “on” signal (1.00) from slightly decreasedsignal strength (0.99), typically from 1 to 0.99. Because a totally “on”signal tends to be noisy, it is easier to differentiate low signals fromzero than it is to differentiate high signals from 1.

Other read out methods including phase conjugate readout, may also bepossible. It is noteworthy to mention that unlike holographic datastorage systems that are intended to store terabits of information in acubic centimeter, in which the writing and the readout systems are bothintegral to a system, this concept allows for the separation of thefunctions of “encoding” and “reading” the data not just between systemhardware, but perhaps over a period of time that might extend to severalyears. Thus, phase conjugate readout may not be as useful as the opticalcomponents may not necessarily constitute the same path.

f) Other Optical Elements and Components

In addition to the storage of digital data onto printed substratematerials, the functions of several types of discrete optical componentscan be imparted onto printed substrate by exposing the substrate to theproper patterns of intensity varying waveforms at the properwavelengths. Lenses, for instance, can be made in a variety of diametersand with a variety of optical powers by controlling the diameter andradial intensity distribution of a laser that is used to expose thesubstrate. Such lenses can be integrated onto printed plastic (e.g.transparent plastic) or glass substrate material to provide fortransparency. For such applications it may be important to minimizescattering of light by the refractive particles suggesting that theyshould be very small. Other optical elements include wavelengthselective mirrors and filters, which can effectively reflect or transmitlight, at specific wavelengths, as a function of the periodicity of thegrating. Still yet other optical elements include waveguides, switches,fibers, optical paths and conduits, periodically polled structures, etc.Though not all optical components have been mentioned, the basicconcepts of printing a layer and exposing it to radiation to create anoptical element would still apply to the device.

In addition to printing a single layer to make a device, multiple layerscould be printed, with different materials for instance, to yielddifferent Δn's or different wavelengths of writing sensitivity (UV andIR). Multiple layered devices could be fabricated to provide multiplelevels of performance in much the same manner as printed circuit boardsprovide multiple channels of conductivity for electrical circuits.

Related Variations or Modifications

In addition to printing the photosensitive material onto substrates, itcould also be sprayed on or the substrate could be dipped/submerged init. Additionally, it will likely be possible to incorporate thegermanosilicate particles directly into the plastic (PVC, PMMA)substrate that some currencies, such as the Australian or Mexicanbanknotes, are made from. This would enable the windowed areas in thesetypes of banknotes to be encoded with gratings. The GeSi particles couldalso be incorporated into plastics such as those that are used forcontainers, which would enable gratings to be encoded into bottles,packaging materials, or other objects.

In addition to storing small amounts of data onto a substrate, it mayalso be possible to store large amounts such as the 9-10 Gbytes requiredfor DVD movies. This would enable for instance an entire movie to beencoded onto a small substrate the size of a standard credit card orsmaller.

Another variation of the invention is the fabrication of printableoptical elements, in particular printable diffractive imaging lensesthat will focus light. In such an embodiment, transparent plastic orglass substrate material would be coated (printed, sprayed, etc.) withthe photosensitive ink, or doped with the photosensitive particles asdescribed previously. A diffractive pattern can be imparted onto thesubstrate material in such a manner so as to create an optical elementthat will act so as to focus incoming light. Such elements are similarto Fresnel lenses, but use a diffraction pattern rather than refractiverings to act on the incoming wavefront. In this manner, lightweight, lowcost planar substrate material can be written with a focal length, andsubsequently cut with an external contour to conform to an outerperimeter shape factor (circular or other shapes to fit into a pair ofglasses or other optical fixture) without having to grind the surfaceshape to a particular curvature. Using diffraction in flexible planarsubstrates to focus multi-wavelength light, rather than surfacecurvature in a mono-refractive media, will substantially reduce the costof manufacturing lenses and other optical elements. Chromaticaberrations can also be reduced with the appropriate phase grating inthe material. Such optical elements can be recorded either by the use ofinterference patterns, or again by the use of scanned laser beams asvery high precision control can be attained with deflectiontechnologies. Additionally, multiple foci can be integrated into thesame element to create bifocal, tri-focal, and multi-focal elements.Writing gratings in such a manner would substantially reduce the costlymanufacturing requirements of many optics, especially those used forvision correction, including contact lenses. The transition between thetwo focal regions could be very smooth, eliminating the sharp edge thatis customarily ground into such corrective optics. The substratematerial could be printed and then stored at low temperature to preventout gassing of the molecular hydrogen, prior to exposure.

Holograms have been incorporated into plastic and paper substrates forsome time for security purposes. They can be found on the packaging ofsoftware and other products, currency, government issued forms ofidentification such as driver's licenses and passports, and even oncredit cards. On currency, for instance, they are first fabricated thenaffixed to the paper with an adhesive. This method of incorporating thesecurity feature requires many additional steps and results in a finalproduct that does not have high integrity. Exposure to heat, water, andother elements degrades the applied feature causing it to delaminate,with machine-washing nearly destroying it. Furthermore, the holographicimages that are incorporated are all the same and do not enable uniquefeatures such as serial numbers or other information to be incorporatedinto each document.

Other forms of holographic media that could be applied directly to asubstrate, such as the polymer made by InPhase (Bayer) Systems, issensitive to ambient light which requires it to be applied in completedarkness and subsequently written in darkness then fixed to preserve theinformation. This process does not readily lend itself to printingcurrency as it is done on large industrial presses in well-lit rooms.

This invention enables holographic recordable ink to be printed directlyonto the substrate, and subsequently exposed to record and encodeinformation onto the substrate. Information so encoded can bemachine-readable and covert, or it can be used to create a visiblehologram that can be readily seen as a virtual image. Exposure toambient room light has little or no effect on the material prior tograting writing, as the wavelength region that it is photosensitive tois narrow, in the UV, and requires substantial intensity of exposure.Such exposure intensities can be readily achieved with a UV laseroperating at the requisite wavelength, and a laser is required togenerate the interference pattern anyway. By using a spatial lightmodulator to impart serialized or document specific information directlyinto the hologram, any grating can be written including a different onefor each document. Current document feeds for currency require 30 notesper second to be written which provides for 0.03 sec exposure. This issufficiently slow to enable phase gratings to be encoded in productionprinting applications.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

1. A method of forming at least a portion of an optical element, themethod comprising: applying a material onto a substrate; exposing aselected portion of the material to electromagnetic radiation, therebyaltering at least one optical property of the material to create theoptical element.
 2. A method as in claim 1 wherein the material is aphotosensitive ink and wherein the applying comprises a printingoperation and wherein the at least one optical property comprises arefractive index and wherein the optical element comprises a grating. 3.A method as in claim 1 wherein the material is a photosensitive ink andwherein the applying comprises a printing operation which prints thephotosensitive ink as a layer onto the substrate, which is transparent,and wherein the optical element comprises a diffractive imaging lens. 4.A method as in claim 4 wherein the optical element is capable offocusing light.
 5. A method as in claim 4, the method furthercomprising: cutting the transparent substrate to conform to an outerperimeter shape.
 6. A method as in claim 5 wherein the diffractiveimaging lens is formed without grinding a surface of the diffractiveimaging lens.
 7. A method as in claim 5 wherein the outer perimetershape corresponds to a perimeter of a frame which is designed to holdthe diffractive imaging lens, and wherein the selected portion is lessthan all of the material.
 8. A method as in claim 1 wherein the materialis a photosensitive ink and wherein the applying comprises at least oneof: (a) printing; (b) dipping; (c) spraying; (d) coating and wherein thephotosensitive ink is loaded with molecular hydrogen and wherein thephotosensitive ink is stored at a temperature below 0° C. prior to theapplying.
 9. A method as in claim 2 further comprising: applying afurther material, which is photosensitive, onto the substrate; exposingthe further material to create a second refractive index to visiblelight which is different than the refractive index.
 10. A method as inclaim 1 wherein the exposing encodes information onto the substrate. 11.An optical element comprising: a photosensitive material; a substratecoupled to the photosensitive material, wherein the photosensitivematerial has been exposed, on at least a portion thereof, toelectromagnetic radiation to change at least one optical property tocreate the optical element.
 12. An optical element as in claim 11wherein the substrate is transparent and flexible and has an outerperimeter shape to conform to a perimeter of a frame for a lens, andwherein the optical element is capable of focusing light.
 13. An opticalelement as in claim 11 wherein the optical element comprises a grating.14. An optical element as in claim 11 wherein only a selected portion ofthe photosensitive material is exposed to the electromagnetic radiationto encode information onto the substrate.
 15. A method of forming aholographic medium comprising: applying a photosensitive material to asubstrate; exposing a selected portion of the photosensitive material toelectromagnetic radiation to record and encode information into thephotosensitive material.
 16. A method as in claim 15 wherein thephotosensitive material is an ink which is applied by printing the inkonto the substrate.
 17. A method as in claim 15 wherein the ink isloaded with molecular hydrogen and stored at less than 0° C. before theapplying.